The most feared letter that could be typed in a URL as the first letter by a parent or peer attempting to use your device to search something. Especially stressful if continued by the letters O and then R.

Slang to represent money

pimp, like calling somebody 'g'

P is a personal blunt

common English slang for 'pence', the sub-monetary unit below the pound.
100p = 1 pound

Pure methamphetamine (speed)

when one is referred to as a "P" that is how drug dealers commonly reference and/or address their customers. More often than not, dealers use the term "P" while talking to or discussing clients that they are somewhat fond of and/or are frequent/familiar customers.